In general, a magnetohydrodynamic (MHD) system uses a liquid metal moving through a magnetic field to produce electricity. Examples of such systems include U.S. Pat. No. 4,749,890 to Houston, in which the fluid is pumped to an elevated collection zone and a motive force is generated by cyclically heating and cooling a portion of the liquid. The fluid moves through the system by gravity.
U.S. Pat. No. 4,599,551 to Wheatley discloses a thermoacoustic MHD electrical generator which includes an irreversible thermoacoustic heat engine coupled to a MHD electrical generator. The engine is positioned in the field of a magnet and is oriented such that the liquid metal oscillates in a direction orthogonal to the field of the magnet, thus producing an alternating electrical potential.
U.S. Pat. No. 4,703,207 to Bodine discloses an alternating current MHD generator in which a pair of resonant combustion chambers are interconnected by a narrow channel. Shock waves formed in the combustion chambers move ionized gas from chamber to chamber. A magnetic field is set up around the narrow channel so that the moving gas produces an alternating current.
U.S. Pat. No. 4,785,209 to Sainsbury discloses a MHD generator having a pair of primary chambers containing a quantity of fluid heated by a heat source. The primary chambers are interconnected at their lower ends by a channel incorporating a MHD cell, and fluid movement is established through cyclic heating and cooling the working fluid.
Conventional reciprocating piston-type internal combustion engines, such as those used in automobiles, require the incorporation of a power transmission to produce work in the form of locomotion. Energy is consumed, and thus efficiency is lost, during the conversion.
Reciprocating engines used to operate electric generators must rely on throttle control (r.p.m.s) to vary electrical output. However, the use of throttle control will require the engine to operate at less than optimal speeds in terms of fuel efficiency and engine wear mode.
The main problem with existing AC generators is that the designs require a constant frequency, which is not suitable for vehicular propulsion without complicated and heavy electric power handling interfaces between the generator and the propulsion motors. A DC generator could be driven by a combustion engine, but then the problem of throttling down the engine to lower speeds results, and there are many problems with high power DC generators involving commutators and brushes. Furthermore, an internal combustion engine combined with a DC generator cannot provide variable power at one frequency unless one introduces the complicated and heavy electric power handling interface.
Magnetic energy may also be converted to electrical energy by linear actuators as disclosed in U.S. Pat. Nos. 3,891,874 to Roters, 4,349,757 to Bhate, 4,602,174 to Redlich, and "Optimal Design of a Tubular Permanent Magnetic Linear Actuator", by S. A. Nasar and C. Chen Electric Machines Power Systems, Vol. 14, pp.249-259, 1988. These linear actuators use an oscillating tube, with permanent magnetic inserts within the tube, to produce the desired electrical power. Linear actuators which incorporate an oscillating metallic tube design have limited stroke lengths. The stroke length is limited by the requirement that the diameter of the oscillating tube must increase as the stroke length increases, and vice-versa. This limitation is further defined by restrictions on the physical dimensions of oscillating permanent magnets constructed of modern high magnetization materials, such as ferrites, which are inherently brittle and lack structural strength. S. G. Carlqvist et al. disclose the necessity for providing an even number of linear actuators operating in opposing motion to substantially prevent vibration of the generator. "Study of 4-Cylinder, Double-Acting and Hermetic Sealed Stirling Engine", Proceedings of the 25th International Energy Conversion Engineering Conference, Reno, Nevada, Aug. 12-17, 1990, Vol. 6, pp.323-328.
The generators discussed above fail to disclose an apparatus capable of providing the variable power at variable frequencies necessary to power a vehicle (i.e., automobiles and trucks).